1. Field of the Invention
This invention relates to the preparation of adhesives, matrix resins for composites, molding powders, films, and laminates, especially those showing enhanced flow with retention of mechanical and adhesive properties.
2. Description of the Related Art
High performance polyimide adhesives are used in the aerospace industry, for example, in joining metals to metals or metals to composite structures. In addition they are rapidly finding new uses as matrix resins for composites, molding powders, and films. These materials display a number of performance characteristics such as high temperature and solvent resistance, improved flow for better wetting and bonding, solvent resistance, high modulus, chemical and hot water resistance, etc. One area of application is in the manufacture of lighter and stronger aircraft and spacecraft structures.
LaRC-TPI is a thermoplastic polyimide well known as a high performance material (V. L. Bell, B. L. Stump, and H. Gager, J. Polym. Sci., Poly. Chem., Ed. 14, 2275 (1976); D. J. Progar, V. L. Bell, and T. L. St. Clair, NASA Research Center, "Polyimide Adhesives," U.S. Pat. No. 4,065,345 (1977); V. L. Bell, NASA Langley Research Center, "Process for Preparing Thermoplastic Aromatic Polyimides," U.S. Pat. No. 4,094,862 (1978); A. K. St. Clair and T. L. St. Clair, NASA Langley Research Center, "High Temperature Polyimide Film Laminates and Process for Preparation Thereof," U.S. Pat. No. 4,543,295 (1985); A. K. St. Clair and T. L. St. Clair, "A Multi-Purpose Thermoplastic Polyimide," SAMPE Quarterly, Oct. 1981, pp. 20-25).
Its structure, as shown below, ##STR1## is derived from 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA) and 3,3'-diaminobenzophenone (3,3'-DABP) and is prepared in 2-methoxyethyl ether (diglyme). It was invented at NASA Langley Research Center and is a commercially available product sold by Mitsui Toatsu (Mitsui Toatsu Chemicals, Inc., New York) and the Rogers Corporation (D.C. Sherman, C. Y. Chen, and J. L. Cercena, SAMPE Preprints, 33, 134 (1988)) as a molding powder, poly(amic acid) solution, and film.
The commercialization of the 3,3'-DABP component of LaRC-TPI has not occurred in the United States because it has been shown to be a mutagen. (W. D. Ross, J. E. Nobel, J. A. Gridley, J. M. FullenKamp, M. T. Winiger, and J. A. Graham, "Mutagenic Screening of Diamine Monomers", NASA Contractor Report 166085, Monsanto Research Corporation, February 1983.) Hence, only experimental samples of this chemical can be purchased for research purposes. One U.S. company prepares and polymerizes the 3,3'-DABP in situ, limiting the exposure of workers to this diamine. However, this process is costly.
In addition, the original version of LaRC-TPI (solution of poly(amic acid) in diglyme) appears to have an undesirably large and(or) broad molecular weight distribution, containing amine and anhydride functional end groups such that the initial imidized powders do not mold well and exhibit poor melt stability.
We have discovered a way of producing high molecular weight 4,4'-isophthaloyldiphthalic anhydride-metaphenylenediamine (IDPA-m-PDA) polyimides (henceforth referred to as LaRC-I-TPI) that have controlled molecular weight through end-capping. These materials are readily processed in their fully imidized form to afford well-consolidated composites, high quality moldings, high strength adhesive bonds, and tough flexible films and coatings.